Coal-fired power plants are utilized throughout the world to generate electricity. Typically, the coal is either in a pulverized form or within a slurry is combusted to generate heat within a boiler to raise steam. The steam is passed into a steam turbine to generate electrical power.
There has been recent interest in capturing carbon dioxide from power plants that use coal and other carbonaceous feed stock such as asphalt, heavy oil, petroleum coke, biomass or natural gas. An integrated gasification and combined cycle (IGCC) is proposed as a preferred method of power generation when carbon dioxide capture is required. In IGCC, gasification of fuel produces a synthesis gas containing mainly hydrogen, carbon monoxide and carbon dioxide with some amount of methane and sulfur and chloride containing impurities. In a typical gasifier the carbonaceous feed is reacted with steam and oxygen to produce the synthesis gas. Typically, the oxygen is provided to the gasifier by a cryogenic rectification plant in which air is rectified within distillation columns at low temperatures to produce the oxygen.
In an integrated gasification combined cycle, the synthesis gas produced as a result of the gasification is cooled to a temperature suitable for its further processing in a water-gas shift reactor to increase the hydrogen and carbon dioxide content of the synthesis gas. The water-gas shift reactor also hydrolyzes most of the carbonyl sulfide into hydrogen sulfide. The synthesis gas is then further cooled for carbon dioxide and hydrogen sulfide separation within a solvent scrubbing plant employing physical or chemical absorption for separation of the carbon dioxide and hydrogen sulfides and carbonyl sulfide from the synthesis gas. This allows for the capture and sequestration of the carbon dioxide which is present within the synthesis gas. The resulting hydrogen-rich gas is then fed to a gas turbine that is coupled to an electrical generator to generate electricity. Heat is recovered from the cooling of the raw synthesis gas stream, from cooling the heated discharge from the water-gas shift reactor, and cooling the exhaust from the gas turbine to raise steam and to generate additional electrical power from a steam turbine.
As can be appreciated, the integrated gasification combined cycle is environmentally very advantageous in that a clean burning synthesis gas stream is used to power the gas turbine while at the same time, the carbon dioxide produced by the gasification can be captured for use in other industrial processes, for enhanced oil recovery or for sequestration. The disadvantage of such a cycle is the high energy penalty associated with the air separation and solvent scrubbing plants. Additionally, the recovery of heat energy in several stages is inherently inefficient in that such heat recovery always involves loss and in any case, the heat is recovered at a low temperature. Lastly, the use of solvent scrubbing plants, water-gas shift reactors and gas turbines is an expensive proposition given their acquisition costs.
It has been suggested to supply the oxygen to the gasification process in lieu of a cryogenic air separation plant with the use of oxygen transport membranes. In such membranes, oxygen is separated from the air with the use of a gas tight membrane formed of the ceramic material that is capable of oxygen ion transport at elevated temperatures. The oxygen ionizes on one surface of the membrane by gaining electrons to form the oxygen ions. Under a driving force of a partial pressure differential, the oxygen ions pass through the membrane and either react with a fuel or recombine to elemental oxygen liberating the electrons used in the ionization of the oxygen.
Where the membrane material is a mixed conductor, the electrons are transported through the membrane. In a membrane that uses an ionic conductor, that is, capable only of conducting oxygen ions, the electrons are transported with the use of an external electric circuit.
The use of ceramic membrane systems have also been contemplated in connection with boilers to generate product steam that has value as a feed to a refinery or to a steam methane reformer as opposed to a combined cycle incorporating a steam turbine. In such boilers, the combustion of a fuel such as natural gas is supported by oxygen separated within the membrane system. Since such combustion will produce a flue gas rich in carbon dioxide and water, the production of a carbon dioxide-rich stream can be accomplished by condensing the water out of the flue gas and then compressing the stream.
An example of a study of such a boiler with carbon dioxide capture appears in a paper entitled, “Cost and Feasibility Study on the Praxair Advanced Boiler for CO2 Capture Project's Refinery Scenario”, Switzer et al. (2005) published by Elsevier. In this paper, a boiler is disclosed in which fuel and recirculated flue gas is passed through a housing having a heat recovery steam generator to exchange heat from a retentate produced by the membrane system with boiler feed water to raise saturated steam. The fuel and flue gas mixture then passes to the membrane system for combustion and generation of the flue gas. Intermingled with the membrane system is another heat recovery steam generator to superheat the saturated steam and thereby to produce the product steam. Heat is recovered from the flue gas to preheat the air and the boiler feed water. Other boiler designs that incorporate oxygen transport membranes and that are capable of producing product steam are disclosed in U.S. Pat. Nos. 6,394,043; 6,382,958 and 6,562,104.
As will be discussed, the present invention provides a method of generating electrical power in which a synthesis gas stream is produced by gasification is then combusted in an oxygen transport membrane to generate heat and in turn raise steam for a steam turbine. As will become apparent, a method of the present invention generates the power in a more thermally efficient manner than an integrated gasification combined cycle with carbon dioxide capture and does not require the use of expensive gas turbines, water-gas shift reactors and solvent recovery units for carbon dioxide capture.